294 NORTH AMERICAN MUSTELIDJE. 



with tlie greater Dumber of species and most general distribu- 

 tion, occurring in both hemispheres. ISearly allied genera, by 

 some considered only as subgenera, are based chiefly upon 

 modifications of the claws, which, in some of the Old World 

 Otters, are small, rudimentary, or even wanting, as in Le]}- 

 tonyx and Aonyx. The most remarkable genus is the South 

 American Pterura or Pteronura, peculiar in the lateral dilata- 

 tion of the tail. 



Lufra itself is the only North American genus of the sub- 

 family, our species having been unnecessarily, if not unwarrant- 

 ably, distinguished by generic name from the European type of 

 Liitra. ♦ 



The Genus LUTRA. (Linn.) 



Generic cuxjiactkrs.— Denial formula : i. |5f; c. yE^} ; i""- 35^3 J 

 m. 2^=^ il"^"^*^' Teeth of ordinary caruivorous pattern. Molar of upper 

 jaw large, quadrate. Back upper premolar with a large internal shelf, mak- 

 ing the contour of the whole crown triangular. Skull much depressed and 

 tlattened on top, the dorsal outline more or less nearly straight and hori- 

 zontal ; rostrum extremely short, bringing the fore ends of the nasals nearly 

 opposite the anterior root of the zygoma, tbe sides of the rostrum erect, the 

 top flat. Cerebral portion of the skull swollen backward, with strongly con- 

 vex lateral outline. Postorbital processes variable (well developed in some 

 species, as in the North American, wanting in others). Anteorbital foramen 

 very large, bounded above by a slender bridge of bone. Posterior nares 

 thrown into one conduit.* Palate extending far back of molars. Ptery- 

 goids strongly hamulate. Body stout, but lengthened and cylindrical; 

 muzzle very obtuse; ears very small. Feet short, broad, naked, or partly 

 hairy on the palm and sole, the digits fnll-webbed. Tail long, tapering, cylin- 

 drico-depressed, but without special lateral dilatation. Claws, though small, 

 well formed. Pelage without striking color-contrasts. 



Many of the foregoing expressions are applicable to the sub- 

 family at large, as well as to the present genus. Particular 

 points of Lutra proper are the presence of perfect claws, in 

 comparison with their absence or rudimentary condition in 

 some other genera, and the lack of special dilatation of the tail. 



The uniformity of coloration and the great individual varia- 

 bility in size throughout this genus render the determination 

 of the species difficult. The points which I have found most 

 available in specific diagnosis, when cranial and dental charac- 

 ters fail, are, the size and shape of the nose-pad and the con- 



* In some species, there is an incomplete septum extending further back 

 than in Mustelince, but never, so far as I know, to the very end of the bony 

 palate, as in Mejyhitinw and MeUme. 



